Semi-conductor device



.sectional view of apparatus United States Patent 2,840,496 Y SEMI-CONDUCTOR DEVICE Dietrich A. Jenny, llrinceton,.N.J-.-, assignor to Radio Corporation of America, a corporation of Delaware Application November'ZS, 1953, Serial No. 394,383 6 claims. .,(cl. 14s-1.5)

operation of these devices is usually subject to relatively severe maximum temperature limitations. ture limitation ofa typical devi bythe energy gap between the duction band of the semi-conductive material of the de- The temperavice. When the temperature of the device is increased t-o a point where thermal energy becomes sutiicient to raise substantial .numbers of the semi-conductive characteristics of thematerial are adversely affected. For example, the energy gap of germaniumrv is labout`0.7 electron voltY and many devices utilizing germanium b ecome inoperative above vtemperatures as lowasSOfC. J v Y Accordingly itisanjobject of the inst provide improved semi-conductive materials.

Another object is to provide improved semi-conductive materials having relatively high energy gaps between their conduction and Vvalence bands. Another object is to provide improved semi-conductor devices operable at relatively high temperatures.

Another object is to provide improved semi-conductive cadmium telluride having n-type conductivity. Another'object is to provide within a body of p-type semi-conductive cadmium telluride a region of n-type conductivity an'd a p-n'rectifying junction between saidregion andthe remainder yof the body.' f f Still another object is toy provide a method of creating a p-n rectifying junction within a body of p-type semicond-uctive cadmium telluride. Y, p

A still further object is to provide an improved method of forming anelectrode 'fused to a body of semi-conductive cadmium telluride having a p-n rectifying junction associated therewith. These and other objects may be accomplished by the having p-n rectifying gap of cadmium telluride is about and devices based on cadmium at relatively hightemperatures.

be described in of which:

The energy 1.45 electron volts, telluride are operable `The .invention will connection with the drawing Figure l is a partially suitable for use in producing nt invention to greater ldetailfin 1 schematic, elevational cross'- utilizing semi- Y telluride. It is` preferred electrons across the energy gap,

Y attected by per'ature gradient along the length vof 2,840,496 ,Patented v June. 24,

Vsemi-Iconductive cadmium'ftellride according to the instant invention. ,l

Figures 2fan`d v3" are schematic, elevational, crosssectional views 'of awafer of semi-conductivev cadmium telluride illustrating successivehsteps a semi-conductor Adevice according to the invention. Q" Similar referencev characters are applied to similar ele-r` ments throughout the drawing. v

Onefeature of of n-type semi-conductive cadmium made utilizing the telluride, which may apparatusI shown in of relatively pure cadmium tellurlde cadmium telluride equivalent quanttiesjof 'cadmium and tellurium in -stoichiometric proportionsimay be used.

About 0.1 %'.by weightofan impurity-yielding material is placedin the tube with the cadmium telluride. This material'gmay klbeanytof the elements ofthe IIIdor VIIa gens.) Conveniently, indium may be used.

Generally, anyjquantity in excess of 10-6 cent of one or more of theelements specified is suicient to impart n-type conductivity to semi-conductive cadmium not 'to `employ relatively large quantities, that is, over a` fewfpercent,

purity-yielding'materials :since the semi-conductive charvt of thecadmium, tellurium and theV impurity-yielding mterial.

The 'Y cadmium telluride fromoneend of the tube to ing p roce'ss Thisv may be the other by a :gradient freezaccomplished 4by slowly withy the charge.` One end 12,.of the charge is removed from effect of the'resistance elements and is maintained Lata slightly lower temperature thanV the remainder`of the charge; Thet temperature of the furnaceV is `gradually reducedto freeze the charge ina progressive longitudinal mannerf l It is desirable during the freezing process to maintain one end to they other as the over-all furnace temperature is reduced. If arelatively steep temperature gradient is provided, the luniformity'lof the frozen ingot may be preferential condensation of vapors at the low temperature end ofthe tube.- l yThis process is called'gradientfreezing and may be Gradient freezaccomplished by any knownapparatus.

i ing assists in'producing ai cadmium tellurideingota'majo in the production of the instant invention is the production` Y A quantityAZ, for example, about v1.0 lgrams isvplace'd within a'V y atomic peri`sthen frozen progressivelyA the direct heating rialby thev elementofcolumn Illa expected and `surprising in. `the crystal structures.

` method, breitiy,

has a' relatively `liighdegree of purity. and" also aV substantial proportion of the portion of which Many impurities impurity-yielding material initially added to the melt are driven by thefgradientfregzingjrprocessto thelastgfrozeri portion of the ingot.

ductivit apparently. oppositeeifect in `cadrriium telluride is`explainable on a basis ofiatornic .substitutions `It is not `intended 5to liniit the by ltliisttieory. Itis believed, ,IIfl substitute scope of theinvenltion hwever, that the'` elements of I col'uriinr themselves in the `crystal` lattice ,ofcadmium telluride more .readily in thefplace ojfcadmiurn than in the place andzthe elements of the I'Ila `groiipjliave three valence i electrons, one electron of'each impurity atom is not bound available `for-conduction. ln

in the" crystal lattice #but is `the caseof the halogens; it isfbelieved thatgthese are i i i the crystal lattice', a

'again withthe freeing of an excess electron.. l

substituted in the place of tellurium in view, the materialsfof the From` a i chemical `point lof tto 'be cdminumrtelluride invention may -be consideredl This eifectdependso c ourseii orithe cegregationcharacteristics ofthesystem.

ralf'relatively' large crystallites', somelofA hich maybe concentration desired will depend upon the resistivity desired in the material, which in turn depends to a large extent upon the particular use to which the material is to be put. For certain devices, for example, sucli as those designed to operate at relatively high frequencies it may The particularY be desirable to provide semi-conductive cadmium telluride lof relatively low resistivity. Other devices may require relatively high The resistivity varies inversely. with the concentration of impurity centers. The concentration of impurity centers may be controlled b y varying the amount of impurityyielding material incorporated in the melt in much the same manner as in the production of other semi-conducingot, produced as `of =tellurium. Since cadmium hastwovalencefelectrons i nii-um halide) Thisanalysis isfbelievedquite proper,

determinative of the conductivitytypeo'frthematerial it is preferredto Aconsider these atoms alone. i i `i.lny cf \tliethalogens `may be dispersedi in` `cadmium Onel `method i of producing semi-conductive cadmium tellurideh-aving halogen atoms dispersed thereinis generally similar toi-thelmethodof making cadmium seleiiide describedin theco-pending application of Richard H. Bube and Soren M. Thomsen, Serial-No. 3495661,V tiled April 20,r 1953, now abandoned, and assignedto thesanie assignee as ,the linstant application. According to this cadmium, tellurium end ahalogen or a halide, all in gaseous states are `interniiired in a closed vessel.. Any one ofthe halogens issuitable,y o r vamixture but since the groupllla or Vila elemwt atoms areprimarily i telluride fto produce ii-typey semi-conductive materials. 1 u

of .morethan one and, alternatively, fa; decomposable halide Vmay be usedsuch as hydrogen or vmethyl chloride or iodide. The temperatureof the mixture is `controlled so that Ias the gases interact solid crystalsof cadmium telluride are`formed drectlylfrom the gases. VThese crystals include `dispersed halogenjatoms. which determine the conductivity type ot the crystals.

4A preferred method of introduci rig halogen impurity centersV into cadmium telluride comprises melting cadmium telluride together. with Va quantityof a cadmium halideand'freezing the mixture thusformed. This meth` odisgenerally `similar tothe method heretofore described-in connection withjdopingicadmum `telluride with indium.` A cadmium halide such as cadmium `iodide 'is utilized in place of indium, ried outin an exactly similar By the use: of a-cadmium manner.

halide relatively large concentr-ations of halogen;at0ms-maygbe=induced into cad-y mium telluride, and semiconductive rriyateriagl o f` relatively low resistivity may be formed. Y For example, when a'bout 10 grams of cadmium-telluride.is-meltediwith about 0.001 gram of cadmium iodide and refrozen as heretofore described theresulting materiallias a r esistivity of less than about 10 ohm cm. i

-N-type semi-conductive cadmium-telluride may be lproduced `according to the; invention having about l0 14to 101! impurity f .centers per cubic centimeter f of volume.

and the process maybe carutilized with cadmium ie-iiuride to fami conductive cadmium telluride having an ductive region7 the form semi-conductor devices having p-rirectifying junctions. These materials,.especially'those of column Illa,

solid 'for liquid at room temperature,' may be alloy `junction for example, the device ls howri in Eig- Which 'are devices such as',

ure 3.

As vshown in iguresZ and 3, a pellet 16 of an impurityyielding material-selected from groupV IlIa, forfexample, -indiumis placed onthe surface of a wafer 18 of p -type semi-conductive cadmiumtelluride. The wafer is `preferably a single crystal andjmay be of 4any `convenient size such as about l/s x Ms x .01 thick. v"fhepellet maybe of any size smaller than the wafer. It may' conveniently be made in the form of a disc aiboutLOS'f in diameterand .005" thick. The wafer and pelletare'heated together about liveminutes. to alloy the pellet into the` wafer and to form the-deviceshowii in' Figure 3.

The dev-ieeis generally similar iriconstructionalA details to corresponding alloy type devices madepf other semiconductive materialsl lt consist-s of the YwaferY ,18 of ptype semi-conductive cadmiumtelluride andan indium electrode 16 formed from the pellet `16 lnd `fused to the surface of the water. YThe farthest point of penetration of the pellet into the wafer is called the; alloy front and isshown by the line 22. During the alloy process a portion of the wafer is dissolved into th'eimolteri pellet and during cooling a recrystallized1 region 20 rich in indium andV an `integralpart of. thewafer is'fornied adjacentthe alloy front. This region has `ri-typei conciiictivity. A p-n rectiiying junction 24 is formed adjacent the' alloy front. Electrical leads2 6 and zlmnayfbe attachedfby non-rectifyirig solder connections 27andf2 'to the electrode and` the wafer respectively. V 'Ihe'fdevice.may be etched, mounted and potted accordingtoithe conventional techniques utilized in conjunction withgerrnaniuni semiconductor devices. i Similar devices may be formed utilizing pellets of other elements of the boron gicup, mixtures ofthese elements, or alloys comprising signicant proportions of them.

There have thus been described improved semi-conductive n iaterials,A devices utilizing thesematerials, and improved methods ofmaking botti theV materials andthe `devices. i i

'W'hat is claimed is: f

1. A semi-conductordevice comprising abody ofsemi- Y I l A ri-type semi-concoriductivity of saidregion being primarily determined by` the presenceinsaid. region lofl impurity centers consisting of Aatoirisof at least olnes element resistivity material.

riodic table according to Mendeleef.

6. A material according to claim 5 in which said selected element is indium.

Estes-ences Cited in the le of this patent UNITED STATES PATENTS 1,751,361 Ruben Mar. 18, 1930 2,208,455 Glaser et al. July 16, 1950 2,582,850 Rose Ian. 15, 1952 2,615,060 Marinace et al. Oct. 21, 1952 2,629,672 Sparks Feb. 24, 1953 2,644,852 Dunlap July 7, 1953 2,697,052 Dacey et al Dec. 14, 1954 

1. A SEMI-CONDUCTOR DEVICE COMPRISING A BODY OF SEMICONDUCTIVE CADMIUM TELLURIDE HAVING AN N-TYPE SEMI-CONDUCTIVE REGION, THE CONDUCTIVITY OF SAID REGION BEING PRIMARILY DETERMINED BY THE PRESENCE IN SAID REGION OF IMPURITY CENTERS CONSISTING OF ATOMS OF AT LEAST ONE ELEMENT SELECTED FROM THE IIIA AND THE VIIA COLUMNS OF THE PERIODIC TABLE ACCORDING TO MENDELEEFF. 